cmd/dist: implement fetch prototype

With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

progress/writer.go

@@ -0,0 +1,60 @@
+package progress
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+// Writer buffers writes until flush, at which time the last screen is cleared
+// and the current buffer contents are written. This is useful for
+// implementing progress displays, such as those implemented in docker and
+// git.
+type Writer struct {
+	buf   bytes.Buffer
+	w     io.Writer
+	lines int
+}
+
+func NewWriter(w io.Writer) *Writer {
+	return &Writer{
+		w: w,
+	}
+}
+
+func (w *Writer) Write(p []byte) (n int, err error) {
+	return w.buf.Write(p)
+}
+
+// Flush should be called when refreshing the current display.
+func (w *Writer) Flush() error {
+	if w.buf.Len() == 0 {
+		return nil
+	}
+
+	if err := w.clear(); err != nil {
+		return err
+	}
+
+	w.lines = bytes.Count(w.buf.Bytes(), []byte("\n"))
+
+	if _, err := w.w.Write(w.buf.Bytes()); err != nil {
+		return err
+	}
+
+	w.buf.Reset()
+	return nil
+}
+
+// TODO(stevvooe): The following are system specific. Break these out if we
+// decide to build this package further.
+
+func (w *Writer) clear() error {
+	for i := 0; i < w.lines; i++ {
+		if _, err := fmt.Fprintf(w.w, "\x1b[0A\x1b[2K\r"); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}